Children and adolescents with all forms of shoulder instability demonstrate differences in their movement and muscle activity patterns when compared to age- and sex-matched controls.

Abstract

BACKGROUND: Shoulder instability is a complex impairment and identifying biomarkers which differentiate subgroups is challenging. There is limited fundamental movement and muscle activity data for identifying different mechanisms for shoulder instability in children and adolescents which may inform subgrouping and treatment allocation. HYPOTHESIS: Children and adolescents with shoulder instability (irrespective of etiology) have differences in their movement and muscle activity profiles compared to age- and sex-matched controls (two-tailed). METHODS: Young people between eight to 18 years were recruited into two groups of shoulder instability (SI) or and age- and sex-matched controls (CG). All forms of SI were included and young people with co-existing neurological pathologies or deficits were excluded. Participants attended a single session and carried out four unweighted and three weighted tasks in which their movements and muscle activity was measured using 3D-movement analysis and surface electromyography. Statistical parametric mapping was used to identify between group differences. RESULTS: Data was collected for 30 young people (15 SI (6M:9F) and 15 CG (8M:7F)). The mean (SD) age for all participants was 13.6 years (3.0). The SI group demonstrated consistently more protracted and elevated sternoclavicular joint positions during all movements. Normalized muscle activity in Latissimus dorsi was lower in the SI group and had the most statistically significant differences across all movements. Where differences were identified, the SI group also had increased normalized activity of their middle trapezius, posterior deltoid and biceps muscles whilst activity of their latissimus dorsi, triceps and anterior deltoid were decreased compared to the CG group. No statistically significant differences were found for pectoralis major across any movements. Weighted tasks produced fewer differences in muscle activity patterns compared to unweighted tasks. DISCUSSION: Young people with SI may adapt their movements to minimize glenohumeral joint instability. This was demonstrated by reduced variability in acromioclavicular and sternoclavicular joint angles, adoption of different movement strategies across the same joints and increased activity of the scapular stabilizing muscles, despite achieving similar arm positions to the CG. CONCLUSION: Young people with shoulder instability demonstrated consistent differences in their muscle activity and movement patterns. Consistently observed differences at the shoulder girdle included increased sternoclavicular protraction and elevation accompanied by increased normalized activity of the posterior scapula stabilizing muscles. Existing methods of measurement may be used to inform clinical decision making, however, further work is needed evaluate the prognostic and clinical utility of derived 3D and sEMG data for informing decision making within shoulder instability.